The present invention concerns glass-ceramic materials having unique physical characteristics whereby their mechanical strength is enhanced by a chilling action. It is particularly concerned with certain glass-ceramic materials having these characteristics and further being characterized by very low thermal coefficients of expansion. The latter are due to a lithium aluminosilicate crystal phase, such as a beta-spodumene solid solution or a stuffed beta-quartz solid solution crystal phase, developed in the material.
A glass-ceramic material is the polycrystalline product of uniform, internal, in situ crystallization of a glass by heat treatment. Depending on the size of the crystals developed, the material may be transparent or opaque. Crystal size is influenced, among other things, by the nature of the nucleating agent, the crystal phase formed, and the degree and extent of heat treatment.
The crystal phase in a glass-ceramic usually predominates over any residual glass phase. For example, in most commercial material, the crystal phase constitutes over 90% of the body. A residual glass phase occurs when certain glass constituents either exceed the stoichiometry of the crystal phase or do not enter such phase. The residual glass phase is usually very different in composition from the original glass. Also, the influence it has on glass-ceramic properties is normally considered to be relatively minor.
Numerous publications, describing specific materials and methods of producing and treating such materials, have followed the introduction of glass-ceramic materials in U.S. Pat. No. 2,920,971 (Stookey) granted Jan. 8, 1960. That patent teaches that the first step in glass-ceramic production is melting and forming of a corresponding glass, usually including a crystallization catalyst or nucleating agent. The glass article formed from the melt is then reheated to initially form nuclei. These, in turn, act as sites for crystal growth as the heat treatment continues and the temperature is raised. Since innumerable nuclei form throughout the glass, the crystals that grow on these nuclei tend to be fine-grained and uniformly dispersed.
Glass-ceramic materials tend to have greater inherent mechanical strength than do the parent glasses from which they are produced. A case in point is the low expansion lithium aluminosilicate glass-ceramics employed in commercial cookware. Thus, flexural strengths, measured on abraded canes of the parent glass, tend to be in the range of 4,000 to 5,000 pounds per square inch (psi). By comparison, similar values, measured on transparent glass-ceramic cane (same chemical composition and abrasive treatment), are on the order of 8,000 psi, and on opaque glass-ceramic cane are on the order of 12,000 psi.
Such increased mechanical strength, taken with the very low thermal coefficient of expansion, has been a major asset of glass-ceramic materials, and, for many purposes, has set them apart from glasses. Nevertheless, there has been a continuous quest for means to further increase the mechanical strength of these materials beyond their inherent values. This has resulted in development of several techniques for strengthening these glass-ceramic materials of inherent low thermal expansion.
One such technique is disclosed in detail in U.S. Pat. No. 3,148,994 (Voss). It involves adding a small amount of fluoride to certain glass compositions to produce substantially increased inherent mechanical strength. However, the effectiveness of this expedient is limited to certain compositions. Also, the resulting surface is not sufficiently smooth for some purposes.
Chemical strengthening of glass-ceramics by ion exchange has been widely studied. A typical method is described in U.S. Pat. No. 4,074,992 (Voss). In accordance with that patent teaching, sodium ions are introduced into beta-spodumene crystals, in a surface layer on a glass-ceramic material, in place of lithium ions. The cost of this extra step is generally considered prohibitive.
Treatment in a sulfur dioxide atmosphere, either before or during the ceramming cycle, has also been proposed. This too involves an extra operation, as well as problems in controlling and exhausting the gas.
Another technique involves forming a laminated article wherein a compressively stressed surface layer encompasses a body or interior portion. The laminae may be similar chemically, but differ in thermal coefficients of expansions, or may densify to different degrees as they cool.
For example, U.S. Pat. No. 3,473,937 discloses applying a low expansion, lead borosilicate glaze to a higher expansion, aluminosilicate glass-ceramic to create a compressively stressed body.
U.S. Pat. No. 3,524,748 (Beall) discloses a strengthened glass-ceramic article in which the interior of the article is characterized by an alpha-quartz crystal phase and the dominant crystal phase in the compressively stressed surface layer is beta-quartz. The article is made by developing a siliceous beta-quartz solid solution throughout and then quenching to prevent a surface layer from changing while the slower cooling interior undergoes inversion to alpha-quartz.
The same principle of a compressively stressed surface is implemented in a rather different manner in U.S. Pat. No. 3,931,438 (Beall et al.). In accordance with this patent, a laminated article is formed with one lamina being a surface layer encompassing the interior. The two portions are similar chemically, and close in thermal expansion, but densify to very different degrees on cooling. This establishes the desired compressive stresses in the surface lamina.